IMO, both collectors are too small and too short. The 1 7/8 can use a 3.5 collector and the 2 can use a 4 inch collector. The smaller header will probably be ok with 18 inch collectors but I'd bet the 4 inch collector wants to be closer to 22-24 inches.
The issue I see is the merge collector. There is science to it, but you can't just slap on merge collectors and expect power. You have to design the system from the intake to the end of the exhaust. Sized properly and with the correct sized valves, a smaller header, with less bends and a merge collector can make more power across the curve and even past peak. BUT, because most headers have bends too close to the port, and too many tight radii, that it's almost impossible at your displacement to get the header too big. I'd also look at the length of the props are tubes. Unless the car is very heavy, or it never sees RPM over 5k then 28-29 inches is about as long as they should be.
The bigger tubes, with the bigger collector and the right length primary tubes should make more power than the smaller headers.